Week 7: Terrain Analysis

• The Earth Surface is not flat

• In addition to X, Y, there is Z (height)
• Also called 3D Analysis

• Different types of terrain - different patterns of X, Y, and Z
• Formed by different processes - geological, hydrological, and volcanic
• Human activities (anthropocene)

Spatial metrics, such as distance, area and volumne

Surface adjusted distance

hydrological modeling

Landslide modeling

Suitability modeling

Two Most Common Terrain Data Models:

• Digital Elevation Data (DEM) (Raster)
• Triangular Irregular Network (TIN) (Vector)

• DEM is basically a normal raster of elevation -it's called DEM because it is specific for a topographic surface
• A DEM is the simplest form to represent a topographic surface
• One of the most widely used data layers in many GIS applications
• The critical parameter is the resolution (or grid cell size)
• 1m resolution DEM for HI, 30m resolution for the US and 1km resolution for the globe are freely available

Essentially a raster of elevation (2D matrix)

DEM in 3D

A vector model for 3D terrain

Delaunay Triangulation between sampling points

A facet of TIN in 3D

Resolution of TIN - number of triangles

• Surface-adjusted metrics (distance and area)
• Slope (Landslide susceptibility)
• Aspect (Solar insolation, vegetation)
• Profile (elevation change along a line)
• Viewshed (visibility)
• Hillshade (visualization)
• Area solar radiation
• Flow path, watershed delination, flow accumulation (hydrology)
• Indices (e.g., terrain ruggedness)

• Convert a vector line to a raster line
• The sum of actual run between every pair of adjacent pixels

• Define sampling points along a transect
• The sum of distance between each pair of points

• Define a number of sampling points along a transect
• The elevation of a point is estimated from the triangle it is located in or the surrounding triangles
• The sum of distance between each pair of sampling points

• ~ 10 different measurement approaches
• Caculation accuracy = f(DEM/TIN resolution, landscape, interpolation method, sampling size)
• Trade-offs between accuracy and data size, accuracy and computation time.

Surface adjusted area is the total area of all the triangular facets

• Angle of the slope (degree)
• Ratio of the slope (percentage)

• In DEM, maximum rate of change in value from that cell to its neighbors.
  

Geometric combination of vertical (x direction) descent and horizontal (y direction) descent

Elevation to Slope

• The direction of the slope

Most GIS packages provide tools for examining the profile of a surface along a selected straight line or series of straight line segments (polyline).

Profile along multiple straight transects

• Profile of a path/route

• Areas visible to a set of observer features.
• Viewshed tool in ArcGIS

• Listening Posts/Observation Posts (LP/LO)
• Weapons placement
• Antenna placement for radar, radios, or cell phones
• Special surveillance equipment placement

• Hypothetical illumination of a surface by determining illumination values for each cell in a raster
• Enhance the visualization of a surface for analysis or graphical display
• Parameters: altitude and azimuth

• The altitude is the angle of the illumination source (e.g. the sun) above the horizon.
• The units are in degrees, from 0 (on the horizon) to 90 (overhead). The default is 45 degrees.

• Azimuth is the angular direction of the sun, measured from north in clockwise degrees from 0 to 360.
• An azimuth of 90 degrees is east. The default azimuth is 315 degrees (NW).
• Why default azimuth is 315 (NW), given most people are living in the North Hemisphere where the Sun azimuth is South?

Sink or hill?

Sink or hill?

Original DEM - hill

• Combining viewshed with elevation

Cumulative solar radiation in a day (Area Solar Radiation tool in ArcGIS)

Solar Radiation: Summer Solstice

Solar Radiation: Winter Solstice

Change of topics: Topic 5: Spatial Interpolation -> Hazard risk and vulnerability assessment Topic 6: Advanced spatial modeling -> Web mapping Next class -> Project discussion

Due date March 17

https://github.com/qiang-yi/GEOG489/blob/master/labs/lab4_terrain_analysis.docx